While the invention is subject to a wide range of applications, it is especially suitable for use with a system for transmitting relatively wideband audio signals, such as 50 Hz to 5,000 Hz voice waves, through two narrowband toll grade lines having, typically, a frequency response of 300 Hz to 3,000 Hz.
There are a number of methods for accomplishing such transmission; for example, Kahn and Gordon U.S. Pat. No. 3,696,298 and Kahn patent application Ser. No. 621,669, now U.S. Pat. No. 4,217,661. Such systems are often implemented with common dial telephone circuits which are prone to failure, especially if used to transmit descriptions of ballgames which continue for extended periods and which must be transmitted over long distances through numerous switching systems, repeaters, etc. One source of unreliability of many dial telephone circuits is the vulnerability of inband signalling circuits. For example, the 2,600 Hz signalling circuit can be falsely activated by certain sounds in speech, such as sibilants. The 2,600 Hz signalling circuit is used to "hang-up" a telephone circuit and ready it for use by a new subscriber. Accordingly, false operation of the 2,600 Hz inband signalling circuit can catastrophically effect a system transmitting information over such narrowband circuits. U.S. Pat. No. 4,018,996 describes a means for greatly reducing such failures.
Another means for upgrading system reliability is to provide redundant circuits and use "voting" circuitry to switch to an alternate line whenever necessary. Voting type equipment is available commercially (Kahn Communications, Inc., Garden City, New York, Line Guard equipment, model LG-2) that monitors two or more circuits transmitting essentially identical signals. By comparison of certain characteristics of the received signals, the equipment can cause associated equipment to switch from a defective or marginal circuit to another circuit providing better performance. However, such equipment is expensive and requires spare communication circuits in order to provide the desired protection against system failure.
While the invention is subject to a wide range of applications, it is especially suited for transmission of broadcast programs from remote sites to broadcast studios and will be particularly described in that connection.